added test for associated Laguerre polynomials (gh-992)

The problem with associated Laguerre polynomials `L_0^(alpha)` yielding `inf` mentioned in https://github.com/scipy/scipy/issues/992#issuecomment-17020819 does not seem to exist anymore. I have added two tests which verify that everything is fine.

Incidentally, I do not see why the problem should be related to the behavior of `hyp1f1` because `b` should be positive while problems with `hyp1f1` only arise for nonpositive integers.